Demand for refund. Is demand for interest valid?

Dear sir We booked a flat with a reputed real estate company in the pre-launch phase and paid 3lakhs as booking amount in September 2014 with the promise that the handing over of the flat would be made in march 2018. We did not see any progress till September 2015 and decided to cancel the booking. However, they have so far delayed refund of booking amount even though they themselves have stated that there are problems with acquiring permissions for construction (which still haven't been obtained till date). They keep making up excuses and have accepted that five other bookings have been cancelled. Even today they say that they are doing a favor by considering our cancellation on priority basis as the agreement says refund six months after cancellation or resale of flat, whichever is later. How can we expect resale when the project isn't even launched. They have cheated many ppl before also, realised this only after booking My queries are 1. Should I go ahead with legal action under RERA? 2. Are we justified in asking for full refund of 3 lakhs along with interest as the deficiency is on their part? Thanking you in advance.